LeetCode 445. 两数相加 II

原题网址：https://leetcode.cn/problems/add-two-numbers-ii/submissions/

数字用链表表示，个位在链表尾；对这两个数做加法，返回新的链表

    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
    

        ListNode list1 = reverseList(l1);
        ListNode list2 = reverseList(l2);

        int mod=0;
        ListNode head = null;
        while(list1 != null || list2 != null) {
    
            int num1 = list1 == null ? 0: list1.val;
            int num2 = list2 == null ? 0: list2.val;

            int sum = num1+num2+mod;
            mod = sum/10;
            ListNode node = new ListNode(sum%10);
            node.next = head;
            head = node;

            list1 = list1 == null ? null : list1.next;
            list2 = list2 == null ? null : list2.next;

        }
        if(mod !=0) {
    
            ListNode node = new ListNode(1);
            node.next = head;
            head = node;
        }
        return head;

    }

    private ListNode reverseList(ListNode head) {
    
        if(head == null || head.next == null) {
    
            return head;
        }

        ListNode cur = head;
        ListNode first = null;
        while(cur != null) {
    
            ListNode next = cur.next;
            cur.next = first;
            first = cur;
            cur = next;
        }
        return first;
    }